Many industrial applications require the use of discrete, well-defined and uniform coatings applied to predetermined areas. Such coatings are very useful in varied processes, such as conformal coatings on non-uniform or irregular substrates like electronic circuit boards. In the production of discrete coatings for application to discrete substrate areas, for example, it is desirable to obtain broad, uniform coatings in a non-contact application process with sharp, square, cut-on and cut-off edges with no stringing of material. In particular, conformal coating material is used to protect selected components of a circuit board from moisture, dirt, etc.
When such coatings are dispensed, the volume of coating material dispensed onto the substrate may be controlled so that substantially the same amount of coating material is dispensed onto successive substrates during production. In one conventional coating material dispensing system, a flow meter is supplied in the line coupling the fluid supply with the dispensing valve. When the dispensing valve is opened, the volume of material dispensed is read using the encoder counts of the flow meter. The time interval during which the measured volume of material was dispensed is also determined and a volumetric flow rate is calculated. This calculated flow rate is compared to a set point representing a desired flow rate and, if necessary, a correction is made to adjust the actual flow rate towards the desired flow rate.
Conventional coating material dispensing systems may be inaccurate if used for applying conformal coating selectively to components or areas of a circuit board because the dispensing valve will only be opened for a very short time interval, perhaps as short as a few milliseconds. During the time interval that the dispensing valve is open, only a very small amount of coating material will be dispensed. The flow meter senses the small dispensed amount as a relatively small number of encoder counts.
Either the time interval, the number of encoder counts, or both, may be characterized by significant inaccuracies, which will result in an inaccurate calculation of flow rate. The system then compares the inaccurate calculated flow rate to the set point to produce an “error.” Because of the inaccuracy, the error from the comparison may result in a correction of the wrong magnitude or even a correction in the wrong direction. Either result may cause an improper amount of coating material to be dispensed the next time the dispensing valve is opened. The result could easily be that the system produces so much inaccuracy it is of little practical value to the user.
Therefore, improved systems and methods for dispensing coating materials are needed that are not susceptible to such inaccuracies in the dispensed amount of coating material.